Having had a bit more think and poke I reckon 14 awg will probably be OK with a 15A fuse in the feed from each battery. The wires on the battery side of the fuses, and the battery terminals, must be well insulated and protected from damage as they are not protected by the fuses. Also, the plugs you use for the batteries must be rated for 15-20A, and the one between the batteries also needs to be 'proofed' against accidental short-circuiting when unplugged, e.g. well shrouded.
If you decide to fuse the motor as well then a 15 should be OK there too. Obviously any sub-circuits for lights, etc, should be fused appropriately for their use/wiring - I think Action do a little board to cater for that as well.
If this sounds overkill just bear in mind that your proposed set up is sufficient to start a car - i.e. It can probably deliver several hundred amps, maybe even a thousand or more, into a fault. The results would not be pretty - the cost of a few fuses is minimal in a model this big (ha'peth of tar, etc
).